Until you can see the vet, you should flush the eye with saline and then dry it well with a clean, dry cloth.
You might be seeing the third eyelid. It only presents itself when there has been a trauma to the eye or the dog has an eye infection.
If the saline doesn't help, you will want to see your vet for a diagnosis and to discuss your treatment options.
If there is an infection present, your vet will probably want to get the dog started on antibiotics.
While it's not an emergency, you don't want to let it go too long, either. Can you move your appointment up?